This invention relates to a process and system for removing carbon dioxide from natural gas in a floating environment, such as on a ship. More specifically, the invention relates to an integrated membrane/adsorbent system for removal of carbon dioxide from natural gas on a ship that houses natural gas purification equipment.
In an LNG (Liquefied Natural Gas) plant, carbon dioxide content in the feed gas stream must be reduced to less than 50 ppmv before liquefaction to avoid formation of dry ice within the system. Commercially this can be achieved by using a solvent absorption process such as contacting the natural gas with an amine solvent to remove the carbon dioxide, which is then followed with the natural gas being sent through a molecular sieve dehydration unit to remove water down to below 1 ppmv.
Depending on the amount of carbon dioxide and the volume in the inlet gas stream, membrane processes have also been used to remove the bulk of the carbon dioxide in front of a downstream amine unit. One of the benefits of this membrane-amine hybrid system is the reduction of the size of amine column that is needed and as well as a reduction in its energy consumption. Adsorption systems have also been used for front-end feed purification for LNG plants. TSA (Temperature Swing Adsorption) processes employing molecular sieves such as 4A or 13X zeolites can remove both carbon dioxide and water from natural gas streams. A growing application for a TSA process is for peak shaving of pipeline gas, where a portion of the pipeline gas is converted and stored as an LNG when demand is low. In the TSA process, the adsorbed carbon dioxide and water in the molecular sieve column are regenerated using a hot purge gas, typically from the feed or the product gas stream. The hot regeneration gas is cooled to knock out most of the water and is then returned to the pipeline. The carbon dioxide removed from the adsorbent, which is not condensable at the cooler temperature, is also returned to the pipeline.
There has been a renewed interest in floating liquefied natural gas (FLNG) systems as a way to develop stranded gas fields, isolated and remote from land. These fields generally are too small for permanent platform installation. An FLNG system will use a ship or barge to house necessary recovery, gas treatment, liquefaction and offloading equipment. Compared to a land based LNG plant, an FLNG system will have a greater need for a modular design to minimize the equipment footprint and weight. An additional challenge for FLNG systems is the effect of sea motion on the performance of processing equipment, especially for systems containing liquid. The removal of carbon dioxide by use of an amine system can be impacted by a loss of efficiency from rocking and tilting of the column internal components. While both membrane and TSA systems have been used commercially in offshore platform installation, nearly no operating experiences for amine systems have been reported for offshore platform applications.
In general, membrane processes that use carbon dioxide-selective polymers such as cellulose acetate can not generate a residue or product stream that meets the specification levels of less than 50 ppmv CO2, as the process is limited by the driving force or the CO2 partial pressure across the membrane. Molecular sieve TSA processes typically can not handle a feed stream with more than 3% CO2, since the size of the adsorbent beds that is required become too large and the necessary regeneration gas flow then becomes prohibitively large. Furthermore, for an FLNG application, there is no existing solution to treat or recycle the effluent regeneration gas, which contains the CO2 removed from the feed stream.
There exists a need to develop an improved process or integrated processes that can remove carbon dioxide and moisture to meet FLNG requirements. The desired processes should be compact and robust, and not susceptible to producing natural gas that is below specification due to winds and waves.